The influence of gravimetric moisture content on studded ... · especially in soccer where studded shoes are used. Recent studies measure studded shoe–surface interactions via either

ORIGINAL ARTICLE

The influence of gravimetric moisture content on studdedshoe–surface interactions in soccer

James Clarke1 • Matt J. Carre1

� The Author(s) 2016. This article is published with open access at Springerlink.com

Abstract It is desirable for the studs of a soccer shoe to

penetrate the sport surface and provide the player with

sufficient traction when accelerating. Mechanical tests are

often used to measure the traction of shoe–surface com-

binations. Mechanical testing offers a repeatable measure

of shoe–surface traction, eliminating the inherent uncer-

tainties that exist when human participant testing is

employed, and are hence used to directly compare the

performance of shoe–surface combinations. However, the

influence specific surface characteristics has on traction is

often overlooked. Examining the influence of surface

characteristics on mechanical test results improves the

understanding of the traction mechanisms at the shoe–

surface interface. This allows footwear developers to make

informed decisions on the design of studded outsoles. The

aim of this paper is to understand the effect gravimetric

moisture content has on the tribological mechanisms at

play during stud–surface interaction. This study investi-

gates the relationships between: the gravimetric moisture

content of a natural sand-based soccer surface; surface

stiffness measured via a bespoke impact test device; and

surface traction measured via a bespoke mechanical test

device. Regression analysis revealed that surface stiffness

decreases linearly with increased gravimetric moisture

content (p = 0.04). Traction was found to initially increase

and then decrease with gravimetric moisture content. It was

observed that: a surface of low moisture content provides

low stud penetration and therefore reduced traction; a

surface of high moisture content provides high stud pene-

tration but also reduced traction due to a lubricating effect;

and surfaces with moisture content in between the two

extremes provide increased traction. In this study a stan-

dard commercially available stud was used and other studs

may provide slightly different results. The results provide

insight into the traction mechanisms at the stud–surface

interface which are described in the paper. The variation

between traction measurements shows the influence

gravimetric moisture content will have on player perfor-

mance. This highlights the requirement to understand sur-

face conditions prior to making comparative shoe–surface

traction studies and the importance of using a studded

outsole that is appropriate to the surface condition during

play.

Keywords Traction � Sport surfaces � Frictionmechanisms � Football � Soccer � Moisture content

1 Introduction

Traction has long been cited as an extrinsic risk factor for

lower extremity trauma injuries in soccer (caused by

excessive loading) whilst being an essential requirement

for athletic performance. Traction, in this article, is defined

as the resistance to motion during studded shoe–surface

contact. The tractional properties of a shoe–surface com-

bination must therefore be within an optimal range [1].

Excessive rotational traction is related to injury risk

whereas translational traction is related to performance as

the player requires sufficient traction to accelerate from the

surface. The relationship between traction, injury, and

athletic performance has stimulated a great deal of recent

research investigating shoe–surface interactions in sport,

& Matt J. Carre

[email protected]

1 Sports Engineering Research Group, Department of

Mechanical Engineering, The University of Sheffield,

Mappin Street, Sheffield S1 3JD, UK

Sports Eng

DOI 10.1007/s12283-016-0220-z

especially in soccer where studded shoes are used. Recent

studies measure studded shoe–surface interactions via

either biomechanical [3–10] or mechanical [11–22]

techniques.

Biomechanical studies are vital to provide loading

conditions and information that focuses on the key mech-

anisms and movements that effect traction and hence can

give the boundary conditions for mechanical tests.

Mechanical test devices have an advantage of creating

objective loading conditions that provide a repeat-

able measure of shoe–surface traction, eliminating the

inherent uncertainties that exist when human participant

testing is employed. Mechanical test devices also provide

an ethical method of testing under real-life loading con-

ditions to provide an understanding of the physical inter-

actions at play during shoe–surface interaction.

In 1986 Frederick [2] highlighted the requirement to

develop repeatable mechanical test methods to quantify a

range of traction that will allow acceptable athletic per-

formance and injury risk. Therefore, numerous and varied

mechanical test methodologies exist that measure traction

at the shoe–surface interface in sporting situations [11].

Methodologies vary in terms of their loading conditions,

driving mechanisms and the measurement of traction they

extract [11, 12, 14–17, 20, 33]. These large variations and

the differing surfaces and shoes tested make a direct

comparison of results difficult and, despite some standard

methodologies being defined for traction, there remains

debate over how an acceptable range of traction can be

determined and defined [11, 12, 14, 26]. Studies often

neglect to provide insight into the traction mechanisms

associated with the shoe–surface system of interest and the

physical interaction between the studded outsole and the

surface. There is, in general, a lack of quantified published

research data relating specific surface properties and trac-

tion, particularly for natural sport surfaces [11, 33].

Natural sport surfaces are complex surface systems

comprising granular soil material (made up of sand, silt,

and clay particles) and a grass plant [31, 32]. The tractional

behaviour of a natural turf system has been found to be

affected by a number of system and environmental vari-

ables—these are reported to include: particle make-up

(termed ‘mineralogy’); ground cover; grass species; grass

root length and density; grass length; compaction; bulk

density; temperature; and moisture content

[23, 24, 28, 30–32]. Although advances in understanding

the behaviour of natural turf systems have been made

[31, 32], the high number of variables makes understanding

the effects of an individual variable on player traction

difficult. Orchard hypothesised a relationship model

between surface traction, surface hardness, ground condi-

tions and stud length which states that surface hardness and

shoe–surface traction decrease with increasing soil

moisture content [28]. This is in agreement with Canaway

and Baker who concluded that moisture content is the

dominant factor controlling surface hardness in natural

sand-based soccer surfaces [23]. A link between the studies

reviewed is the controlling effect moisture content has on

soil strength and failure mechanisms. There remains,

however, a gap in knowledge understanding how individ-

ual surface parameters, such as moisture content, affects

the traction a studded soccer boot is likely to provide a

player.

As opposed to comparing multiple shoe–surface com-

binations, the aim of this paper is to understand the effect

that the moisture content of a natural turf system has on the

tribological mechanisms at play during stud–surface

interaction. Therefore, mechanical testing has been used in

this work to provide an objective comparison between

natural turf samples.

2 Method

Six samples of a natural soccer surface (0.5 9 0.5 m) with

a mixed sand, silt and clay make-up were removed from

Norton Playing Fields, Sheffield, and brought into the

laboratory. The samples were taken from the same area of

the playing field to keep surface variables such as grass

cover and length, root density, and soil particle make-up as

consistent as possible. Soil analysis and mechanical tests

(traction and impact) were carried out for each turf sample.

The mechanical tests used an adidas World Cup stud,

designed for the forefoot of the adidas World Cup soccer

shoe (first released in 1954 and still popular at the time of

writing). This is a metal stud and was chosen as a market

leading typical stud designed for use on a natural surface

(see Table 1). The tests conducted on each sample are

described in detail below.

2.1 Soil analysis

2.1.1 Gravimetric moisture content (%)

When evaluating standards for natural soccer pitch surfaces

Baker found that the gravimetric moisture content in a

mixed soil sand-based natural soccer surface varied

between 15 and 45 % throughout a soccer season in the UK

[24]. Canaway and Baker commented that hardness fell

below a preferred limit when the moisture content was

within a range of 34–39 % [23]. Therefore, for this study it

was assumed that a surface sample with gravimetric

moisture content greater than 34 % would be considered

too soft, and hence unplayable. Before any mechanical

testing the six samples were individually prepared to

ensure they would all be in a condition considered playable

J. Clarke, M. Carre

for soccer, but have varying gravimetric moisture contents

(see Table 2).

Cylindrical soil cores with approximately 15 mm

diameter and 50 mm length were taken from each turf

sample to determine their gravimetric moisture content.

Five cores were taken after the sample had been prepared

but prior to any mechanical tests (which are described in

Sect. 2.2), and five cores were taken immediately after the

mechanical testing. Hence ten cores were taken in total per

turf sample. Once taken, the mass of each core was

immediately recorded. Each core was then oven-dried at

110 �C for 24 h and the gravimetric moisture content was

determined as the mean percentage weight loss of each

core.

2.1.2 Bulk density (g/m3)

Bulk density was determined as the ratio of the oven-dry

mass of each cylindrical core and the volume occupied.

Bulk density is regarded as a measure of soil compaction

for soils with the same moisture content and/or soil com-

position. Increasing the bulk density increases the resis-

tance to penetration which is related to the soil’s stiffness

and compressive strength [31, 32]. It can therefore be

assumed that a surface considered to have a low bulk

density will allow improved stud penetration, but reduced

resistance to motion during stud–surface contact due to it

having less material to overcome and move through.

2.2 Mechanical tests

2.2.1 Normal impact tests

The SERG drop hammer, a surface testing device devel-

oped by Carre et al. was developed to determine the normal

impact characteristics of sport surfaces [25]. The SERG

drop hammer (Fig. 1) is based on the Clegg impact ham-

mer, which is commonly used to survey the hardness of

sport surfaces [25].

For this study, the profile of the drop hammer was

modified to include a stud as opposed to having the

hemispherical profile described in Carre et al. this allowed

an investigation into the penetration characteristics of stud–

surface combinations. The hammer contains an

accelerometer and is dropped normally onto the surface.

The device outputs a signal trace from the accelerometer

throughout the entire time of an impact.

For this study the equipment had modifications to that

discussed by Carre et al. the voltage signal from the

accelerometer was sampled via a data acquisition device

(National Instruments model number NI USB6008) in real

time at 10,000 Hz and displayed in LabView (version 7.1

National Instruments). The result is a recorded output trace

of signal voltage against time, throughout the loading and

unloading phase of each impact (see Fig. 2). The unloading

phase describes the contact between the hammer and the

surface following the peak impact force. The signal was

Table 1 Dimensions of studs

used in testingL (mm) D1 (mm) D2 (mm)

Cross-sec�onal area (CSA)of stud profile (mm2)

13 19 9 170CSA

D1

L

D2

Table 2 Summary of gravimetric moisture content and bulk density of each sample

Sample reference Gravimetric moisture content (%) Bulk density (g/m3) Preparation prior to mechanical testing

Mean St dev. Significant differences Mean St dev.

A 13.54 1.47 C, D, E, F 1.16 0.08 Oven-dried at 110 �C for 4 h

B 15.23 5.83 D, E, F 1.21 0.07 Oven-dried at 110 �C for 2 h

C 17.41 4.41 A, D, E, F 1.23 0.15 Allowed to dry indoors for 3 h

D 21.68 1.97 A, B, C, F 1.26 0.05 Kept outdoors and watered

E 23.03 2.59 A, B, C, F 1.31 0.10 Kept outdoors and watered

F 28.39 2.80 A, B, C, D, E 1.22 0.07 Kept outdoors and watered

The influence of gravimetric moisture content on studded shoe–surface interactions in soccer

sampled and transformed into force and displacement.

From the raw trace data the acceleration during an impact

can be found. Using the relationship F = ma, a force dis-

placement plot can then be formed. If the hammer is

assumed to be rigid compared to the surface then it can be

considered equivalent to a force deflection curve for the

surface and can be related to surface stiffness.

Figure 2 shows a typical force displacement–curve

(hysteresis loop) for an impact and highlights the param-

eters found for each trace in order to compare the charac-

teristics of different surfaces. Plotting the hysteresis of an

impact is a useful method to visually inspect and compare

the normal impact characteristics of different surfaces.

Parameters of interest from a hysteresis loop are as follows:

Peak impact force from loading The peak impact force

(F(max)) is a measure of the maximum deceleration of the

hammer during impact. Higher-impact decelerations sug-

gest a surface is harder.

Average stiffness during loading phase This parameter

(K(ave)) is the ratio of maximum penetration force over

deflection of the surface at this point in time. It will relate

to the resistance of penetration by the stud. By inspecting

the loading phases of repeated impacts at different loca-

tions of a surface the mean average stiffness of the surfaces

can be compared.

Ten repeated drops at the same drop height were per-

formed on each sample. The peak impact force and average

stiffness throughout the loading phase of each drop were

determined. The results from these tests were used to

understand the penetration characteristics of each turf

sample. Regression analysis was used to determine the

relationship between the drop hammer test results and the

gravimetric moisture content results.

2.2.2 Traction tests

A bespoke mechanical traction testing device (Fig. 3)

developed by Kirk was used to measure translational

traction [26]. The test device measures the translational

traction developed at the shoe–surface interface when a

horizontal shear force is gradually increased. This is a

valuable simulation as it provides a characteristic of the

surface relative to the traction a player can experience—the

force required to accelerate from a surface during a fore-

foot push-off before a slip occurs [26].

A hydraulic ram provides a controlled constant vertical

load applied to a studded outsole. A high-pressure pneumatic

ram provides a dynamic, increasing force in the horizontal

direction. The horizontal load is controlled by a solenoid

valve which is gradually opened to increase the force in the

cylinder of the pneumatic ram. Load cells in the horizontal

Fig. 1 SERG drop hammer setup and studded hammer profile and hemispherical hammer profile, modified from Carre et al. [25]. (Note: the

studded profile presented here is not that used for testing—see Table 1)

0

500

1000

1500

2000

2500

3000

3500

0 0.002 0.004 0.006 0.008 0.01

Displacement (m)

Forc

e (N

)

F(max)

K(ave)

LoadingPhase

UnloadingPhase

Fig. 2 Parameters recorded from a force–displacement curve to

characterise the behaviour of the drop hammer–surface impact

J. Clarke, M. Carre

direction and a Linear Variable Differential Transformer

(LVDT) provide the necessary data to measure traction—the

horizontal force resisting motion. Voltage signals from the

load cells and LVDT are sampled simultaneously, via strain

indicator boxes and a data acquisition device (National

Instruments model number NI USB-6008), in real time and

displayed in LabView (version 7.1 National Instruments).

The signals are sampled and transformed into force and

displacement measurements.

In this study, the traction rig was placed over a tray

containing one of the natural turf samples and secured to

prevent unwanted movement, shown in Fig. 3. This pro-

vided the device with rigidity and limited deviation in the

applied normal force, a potential issue as reported by

Severn et al. [11]. The tray containing the turf sample was

also securely clamped into position.

The test device was developed to represent a soccer

player performing a forefoot push-off, a movement which

requires sufficiently high traction. The segment of a soccer

boot outsole in front of the metatarsal-phalangeal (MP)

joint is the only section in contact with the surface during

the critical phase of a forefoot push-off [27]. Figure 4

shows that on the studded outsole of the adidas World Cup

boot, the front two studs will therefore be in contact with

the surface. During traction testing the studs were config-

ured in this two-stud formation within a stud plate of the

same dimensions, as shown in Fig. 3. The dimensions of

the studs are shown in Table 1.

For each traction test a vertical force of 350 N was

applied to the stud plate. This was found by Kirk to be

representative of the vertical force (ground reaction) that

occurs during the instant a player is at most at risk of

slipping during a forefoot push-off [26]. The horizontal

force after an initial displacement of 10 mm was used as a

measure of traction. Kirk et al. showed that a surface

‘gives’ by approximately 10 mm during a movement when

the player does not slip [27]. It can be argued that a hori-

zontal displacement greater than 10 mm would result in the

perception of a loss of traction (i.e. a slip). Traction tests

were repeated five times at different location on each turf

sample and the mean and standard error were determined.

2.3 Statistical analysis

PASW Statistics for Windows (Version 18.0 SPSS Inc.

2009) was used to perform statistical tests. One-way

analysis of variance (ANOVA) tests were performed to

determine statistically significant differences between two

data sets. ANOVA tests assume normal (or Gaussian)

distribution and produce a p value (the probability data sets

are significantly different) to assess the statistical signifi-

cance between two data sets. In this study, it was decided

that if p\ 0.05 then the data were significantly different.

Fig. 3 Left: photograph taken during laboratory-based traction testing. Right: stud plate and configuration

Approximate line of MP joint flexion across the boot

1st metatarsal head

5 th metatarsal head

Fig. 4 The studded outsole of the forefoot segment on an adidas

World Cup soccer boot, to show the studs in the forefoot segment

which are likely to be in contact with the surface during forefoot

push-off

The influence of gravimetric moisture content on studded shoe–surface interactions in soccer

The square of the Pearson correlation coefficient was

used to determine the strength of the linear correlation

between data sets. As the coefficient tends to 1 the stronger

the relationship is considered. The corresponding p value

(the probability that the relationship is significant) was

used to determine if the linear relationship was statistically

significant. In this study, if p\ 0.05 then a significant

relationship between the two data sets is said to exist.

3 Results

Table 2 summarises the preparation procedures and also

presents the mean moisture contents achieved (ranging

between 13.5 and 28.4 %) and the bulk density test results

after each sample was prepared. Figure 5 shows the

moisture contents from the five cylindrical cores taken

before testing and the five cylindrical cores taken after

testing. The testing procedure lasted approximately 1 h and

no significant differences were found between the gravi-

metric moisture content between the cylindrical cores taken

before and after testing. Table 2 presents the significant

differences between the gravimetric moisture content of the

samples. It was assumed that the bulk density would be

relatively constant as each sample was taken from the same

section of the pitch. Although differences in the mean bulk

density were found, ANOVA tests revealed no significant

differences (p = 0.259) between the bulk density of any of

the surface samples.

Representative force–displacement curves during the

loading phase from the drop hammer tests are shown in

Fig. 6. Surface A can be described as being a stiffer surface

than surface F. Figures 7 and 8 plot the mean gravimetric

moisture content (%) against the peak force (N) and the

mean average stiffness (N/m) (during the loading phase),

respectively. The plots reveal how as the gravimetric

moisture content decreases the peak force and the mean

average stiffness increase.

Each traction test produced plots of horizontal force (i.e.

traction) against horizontal displacement. Typical plots

over 15 mm of displacement are shown in Fig. 9. Fig-

ure 10 presents a plot of mean horizontal force (N) against

mean gravimetric moisture content (%) for each of the six

samples and are presented in three groups (1–3).

4 Discussion

4.1 Discussion of normal impact test results

Figures 7 and 8 show significant linear negative relation-

ships between peak impact force and gravimetric moisture

content (p = 0.006) and between average stiffness and

gravimetric moisture content (p = 0.04), confirming an

increase in penetrability with an increase in moisture

content. This finding is in agreement with Canaway and

Baker who found moisture content to be the dominant

factor controlling surface hardness, and with Orchard, who

concluded that natural surface hardness decreases as

moisture content increases [23, 28].

Based on a study by Ayers and Perumpal, Jennings-

Temple extracted an idealised curve for the change in

penetration resistance with increasing moisture content

[29, 30]. The curve shows how penetration resistance

0

5

10

15

20

25

30

35

A B C D E F

Sample Reference

Gra

vim

etric

Moi

stur

e Co

nten

t (%

)

Before TestingAfter Testing

Fig. 5 Mean gravimetric moisture contents of the surface samples (±1 standard deviation)

J. Clarke, M. Carre

increases, peaks, and then decreases with increasing

moisture content. In soils, low water content can increase

the yield strength and the elastic moduli of the soil as films

of water under tension pull particles together [26, 31, 32].

Guisasola et al. report that, at low water content, negative

pore pressure causes cohesion from water films under

tension at particle contact [31, 32]. However, the addition

of further water increases pore pressure reducing the

cohesion between particles, and reducing the shear and

yield strengths of the soil [26, 31, 32]. The addition of

moisture also acts as a lubricant during stud–surface con-

tact and at high moisture content the effects of bulk density

can be ignored [31, 32]. The results from this study shows

that the bonds that held the soil particles together have

weakened with increased moisture content; also, that the

moisture has acted as a lubricant reducing the friction force

required for the soil particles to move past each other as the

stud clears a path vertically into the surface.

Fig. 6 Representative force–

displacement plots from each

surface sample during the

impact phase of loading using

the SERG drop hammer with

studded profile

Fig. 7 Plot of mean peak impact force against mean gravimetric moisture content (±1 standard error)

The influence of gravimetric moisture content on studded shoe–surface interactions in soccer

Fig. 8 Plot of mean average stiffness against mean gravimetric moisture content (±1 standard error)

Fig. 9 Representative plots of horizontal force against horizontal displacement for each surface sample using the traction testing device

J. Clarke, M. Carre

4.2 Discussion of traction test results

Figure 10 shows that the surface moisture content has a

significant effect on traction. The surface with the highest

moisture content (sample F) fails under a relatively low

horizontal force, despite having higher stud penetrability. It

was observed that full stud penetration was not possible on

samples A, B and C during traction testing. The relatively

low traction of these surfaces can be attributed to their poor

penetrability. The samples (D and E) which offered the

highest traction were the samples which had a soil strength

that allowed a desirable balance between stud penetrability

and resistance to horizontal force. This suggests that the

adidas World Cup Stud is better suited to samples D and E

as they will provide the player with the highest traction

during a push-off. Future work should be done to fully

substantiate this suggestion through further testing of sur-

face samples.

The findings show how results from the traction rig

could be misinterpreted. Although a result may show stud–

surface combinations to provide similar levels of traction

(i.e. comparing groups 1 and 3), the penetration charac-

teristics could be extremely different and this will influence

a players comfort and stability [34]. This highlights the

requirement to understand the influence of surface condi-

tions when conducting investigations into shoe–surface

traction.

In terms of the mechanical interactions that occur during

testing, initially the traction rig quasi-statically loads the

surface vertically via the hydraulic ram. Then the surface is

horizontally loaded until surface failure occurs; in hard dry

soils brittle shear failure occurs, while in softer soils shear

failure occurs as a result of internal particle flow. The

traction mechanism that describes how the surface fails for

each of the groups of samples circled in Fig. 10 (1–3) are

discussed in Sects. 4.3 and 4.4.

4.3 Dry soil traction mechanism

In group 1 (the samples with mean gravimetric moisture

content from 13.5 to 17.4 %) it can be hypothesised that

brittle failure occurs during vertical and horizontal loading.

High resistance to penetration is due to high particle

cohesion increasing resistance to compression and

increasing its shear strength. This results in comparatively

poor stud penetration as highlighted in Fig. 8. In stronger

dry soils brittle failure occurs in the horizontal causing

shearing along a slip plane. Particles above the slip plane

are ruptured and displaced by the force driving the stud.

This causes a build-up of surface material which may come

into contact with the stud plate. This may result in an

increase in traction as the movement develops and may

explain why Orchard found peak traction recorded during a

large movement increased with decreasing moisture con-

tent [28]. However, this study challenges this finding as

only the initial movement of the stud is relevant to a soccer

player, and it may be that this build-up has a negligible

effect. Despite low moisture lubrication, and hence high

inter-particle friction, the traction force is mainly affected

by the low stud penetration, therefore the ploughing trac-

tion force and skin friction are relatively low and the plate

friction is not contributing to the overall traction force. A

0

50

100

150

200

250

300

350

400

450

500

0 5 10 15 20 25 30 35 40

Gravimetric Moisture Content (%)

Hor

izon

tal F

orce

(N)

High PenetrationLow Traction

High PenetrationHigh Traction

Low PenetrationLow Traction

1

2

3

Fig. 10 Plot of mean horizontal traction force against corresponding mean gravimetric moisture content (±1 standard error)

The influence of gravimetric moisture content on studded shoe–surface interactions in soccer

diagram explaining the traction mechanism is presented in

Fig. 11. In this mechanism initially the traction is domi-

nated by the soil particles resistance to shearing. As the

stud moves the mechanism will also be dominated by the

strength of the surface ahead of the stud, and the friction

forces generated as surface materials interact.

Brittle failure along relatively horizontal shear plane

Surface displacement may cause plate-surface contact further along movement

Horizontal Loading

Surface shear failure and displacement

Vertical Loading

Surface displacement as surface shears under

vertical loading

Plane of shear rupture under vertical loading This traction mechanism is dominated by: Cohesive shear strength in this region Inter particle friction in this region Shear strength in the horizontal plane

Vertical Loading

No outsole-surface contact

Poor stud penetration

Fig. 11 The traction mechanism for surfaces with low moisture content (group 1). Top Surface failure under vertical loading. Bottom Surface

after horizontal loading

Vertical Loading

Surface displacement as surface shears under

vertical loading

This traction mechanism is dominated by: Yield strength in this region Inter particle friction in this region Cohesive shear strength Outsole-surface friction force Stud-surface friction

Outsole-surface contact (full penetration)

Plane of shear rupture under vertical loading

Skin friction force

Surface plastic-deformation

Outsole-surface friction force

Horizontal Loading

Shear failure along relatively horizontal shear plane

Stud ‘flows’ through surface particles at high moisture content. Vertical Loading

Fig. 12 The traction mechanism at higher moisture content (groups 2 and 3). Top Surface failure under vertical loading. Bottom Surface after

horizontal loading

J. Clarke, M. Carre

4.4 Moist soil traction mechanism

In soils a small addition of water can cause a transition

from brittle failure to plastic flow failure [31, 32]. There-

fore, in groups 2 and 3 (with mean gravimetric moisture

contents from 21.7 to 28.4 %) surface failure may occur

due to plastic yielding, as opposed to a brittle shearing.

This failure mechanism is explained in Fig. 12. In group 2

the additional moisture content reduces particle cohesion

and provides a lubricating effect which reduces inter-par-

ticle friction. The resistance to stud penetration via vertical

loading is therefore lower than in group 1 (as shown in

Fig. 10). As the stud vertically shears the soil during

penetration, the pores of water will be broken and, via

lubrication, reduce the skin friction forces and inter-parti-

cle friction forces resisting the stud from vertically clearing

a path. In group 2 the high penetration characteristics will

have contributed to the higher traction forces measured.

Assuming the entire stud had penetrated the surface, a

combination of the three contributing components of trac-

tion (ploughing traction force, skin friction, and plate

friction) would explain the significantly higher traction

forces measured in this group. In groups 2 and 3, assuming

the moisture content had caused a transition to plastic flow

failure, the traction mechanism is dominated by the inter-

particle friction as the stud clears a path through the sur-

face; however, the skin friction and plate friction compo-

nents will also contribute. It is possible that the relatively

high bulk density value (although not significant) could

have contributed to the traction measured.

The low traction force observed in group 3 will be due

to the additional moisture content. The surface fails as a

result of low resistance to particle flow; the high moisture

content will result in poor particle cohesion and hence low

soil shear strength. Despite having high penetration char-

acteristics, when horizontally loaded, the inter-particle

strength and stud–surface friction forces are significantly

lowered by the lubricating effect of the moisture.

5 Conclusions

In soccer it is desirable for the studs of shoe outsole to

penetrate the sport surface and provide the player with

traction to push off from the surface.

An adidas World Cup soccer stud was attached to a drop

hammer which was used to assess the penetrability of a

natural turf surface at differing gravimetric moisture con-

tents. Significant linear relationships were found to exist

between the gravimetric moisture content with the peak

impact force and the average stiffness from the impact

phase of the drop hammer. As the gravimetric moisture

content of a natural turf surface increases, the peak impact

force and average stiffness decreases.

Mechanical traction testswere conducted to determine the

influence the gravimetric moisture content of a natural turf

surface has on the traction a soccer player might experience

during play. Low gravimetric moisture content results in low

stud penetration and hence low traction. High gravimetric

moisture content results in high stud penetration but low

traction, as the moisture acts as a lubricant. High traction is

achieved when the moisture content of a surface is in

between the two extremes, as the surface is sufficiently weak

to allow stud penetration but sufficiently strong to provide

horizontal resistance as the player pushes off.

It is dangerous to make conclusions on the performance

(i.e. stud penetration and traction) of studded shoes based

on a mechanical measure of traction alone. An under-

standing of the condition of the surface is required; this

study shows that significant differences in gravimetric

moisture content may not lead to significant differences in

traction but will have an effect on stud penetrability and

hence player performance.

Acknowledgments The authors would like to thank: the EPSRC for

funding this work; adidas for supplying footwear; and the staff at

Norton Playing Fields, Sheffield, for their help in providing surface

samples.

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